1. Field of the Invention
This invention is directed to an electronic ballast system for fluorescent or gas discharge tubes. In particular, this invention relates to automatic gain controlled ballast systems for fluorescent tubes. Still further, this invention is directed to an electronic ballast system which limits the voltage output to fluorescent gas discharge tubes when the output voltage of the power source increases. The output voltage of the electronic ballast system is also substantially reduced when the gas discharge tube or fluorescent tube is electrically removed from the overall circuit. More in particular, this invention pertains to a no load protection transformer being series coupled to an induction circuit for preventing the generation of voltages above a predetermined value when the gas discharge or fluorescent tube is electrically removed from the circuit. More in particular, this invention relates to an electronic ballast system where a primary winding of a no load protection transformer forms a variable inductance which is inversely proportional to the magnitude of the power delivered to the gas discharge or fluorescent tube. Power is delivered to the gas discharge tube from a center tapped high voltage secondary winding wherein the center tap is capacitively coupled to the power supply reference. Thus providing DC isolation for the secondary circuit while providing an AC coupling to the power supply reference.
Still further, this invention directs itself to a transistor switching mechanism which is feedback coupled to an induction circuit for establishing a regulated current for a gas discharge tube independent of the gain of a transistor used in a particular ballast system unit. Additionally, this invention pertains to an electronic ballast system where there is provided a transistor switching network including a regulation control circuit for maintaining the power output of a gas discharge or fluorescent tube at a predetermined and substantially constant value. The regulation control circuit being coupled in series relation between the DC power supply and the no load protection transformer for monitoring the regulated current without being affected by various noise signals which may be superimposed on the regulated current by the no load protection transformer.
Further, this invention directs itself to an electronic ballast system having a regulation control circuit including a toroidal transformer to provide a predetermined variable inductance for regulating a power output to the gas discharge or fluorescent tube. Still further, this invention relates to an electronic ballast system which is transistorized and where the current gain of various transistors range over a wide value range from one system unit to another and provides for electrical circuitry which will maintain the gas discharge tube light output fluctuation to a minimum tolerance value.
2. Prior Art
Electronic ballast systems for gas discharge or fluorescent tubes are well-known in the art. The best known prior art to the Applicant includes U.S. Pat. Nos. 4,733,135; 4,587,461; 4,503,362; 4,277,726; 4,259,616; 4,075,476; 3,878,431; 3,638,070; 3,619,713; 3,222,572; and, 4,353,009.
With the exception of those prior art systems commonly owned by the Assignee, removal of gas discharge or fluorescent tubes from the ballast circuit causes excessive voltage outputs to be generated at the output terminals. This condition can have a deleterious effect to the operating life of the particular ballast system.
In some prior art systems, such as provided in U.S. Pat. No. 4,733,135, the self-regulation circuitry is coupled between the no load protection transformer and the induction transformer for monitoring the current therethrough. However, the high voltage high frequency signal induced in the secondary of these no load protection transformers tends to reflect transient current pulses into the primary winding wherein it becomes superimposed on the regulated current signal. Thus, in such prior art systems the self-regulating control circuitry attempts to control the base current for the switching transistor in response to the instantaneous additive signal formed by the regulated current and the transient noise. Further, in these prior art systems a DC component of the gas discharge excitation current is coupled to the power supply reference, which adds a second source for noise signals to be introduced into the ballast system.
The instant invention overcomes these deficiencies by monitoring the regulated current prior to its passage through the no load protection transformer circuit and provides a no load protection circuit having a center tapped secondary high voltage output winding capacitively coupled to the power supply reference to isolate the DC component while still establishing an AC reference.